As a result of their increased performance in terms of specific power and power density, nowadays synchronous machines with permanent magnets have extensive application in the field of motor vehicles.
These electrical machines can be produced in an extensive range of powers and speeds, and have applications both in vehicles of the “all electric” type and in vehicles with low CO2 emissions of the so-called “mild-hybrid” and “full-hybrid” types.
“Mild-hybrid” applications generally concern electrical machines of approximately 8 to 20 kW, for example an electric motor which is fitted on the front surface of a thermal engine, and is coupled to the latter by a drive belt. With an electric motor of this type it is possible to reduce the capacity of the thermal engine (“engine downsizing”) by providing electric torque assistance which supplies supplementary power, in particular when revving up. In addition, traction at low speed, for example in an urban environment, can also be ensured by this same electric motor.
Applications of the “full-hybrid” type generally concern 30 to 50 kW motors for architectures of the series and/or parallel type, with a level of integration which is more accomplished that the electric motor(s) in the traction chain of the vehicle.
The remarkable performance levels of the present machines with permanent magnets are to a large extent due to the development of rare-earth magnets of the neodymium—iron—boron (NeFeB), samarium—iron (SmFe), or samarium—cobalt (SmCo) type, which can have residual magnetism in excess of Tesla level.
However, machines with permanent magnets comprising a rotor with a so-called “flow concentration” structure had long since made it possible to obtain substantial magnetic flows using magnets with lower residual magnetism, for example magnets obtained from sintered or bonded ferrites.
Also long since, the dimensional and magnetic characteristics of this type of structure have been optimised, either by carrying out many tests, or, more recently, by carrying out computer simulations, such as to improve the electrical performance of the machines.
An example of dimensional optimisation of the magnets and magnetic poles of a rotor with permanent magnets was disclosed in 1971 in the patent of invention FR 2.084.279.
The dimensional optimisation of the magnets has recently become the subject of attention once more as the result of an increase in the price of rare earth magnets, associated with an unfavourable geo-political situation.
Since the implementation of rare earth magnets in a rotor of an electrical machine designed for motor vehicle applications is no longer economically viable, and probably not sustainable, the other alternative consists of magnets based on ferrites.
However, since the residual magnetism or induction of a ferrite is lower than in the case of a rare earth magnet, it is necessary to increase the volume of ferrite of the magnet in order to obtain an equivalent magnetic flow.
With this magnetic constraint being imposed, it will be appreciated that the volume of the ferrite magnets cannot be increased indefinitely in a rotor with a given size.